The present invention relates to a digital signal transmission system, more particularly, to a transmission system favorable for the improvement of the S/N ratio at the time of transmitting digital signals by light.
Such kind of systems have been disclosed, for instance, in U.S. Pat. No. 4,259,746. In case of transmitting digital signals by light in the system, a light emission diode on the transmission side sends out light all the time at the level value between "1" and "0" in average, without taking it into consideration that the S/N ratio should be made higher by lessening the light emission time of the light emission diode so as to increase the peak light emission amount.